Is clutch disk alignment with the pressure plate always necessary (with a dedicated tool or maybe by other means) for every car model, when replacing a 3 pieces clutch kit?Or are there any cars for which this step can be skipped?

You don't need to align the clutch at all if you don't plan on installing the transmission.

Otherwise, I wouldn't skip it.

The car is a Nissan Almera N16 2.2 Di.The mechanic says they don't use an alignment tool, when installing a new clutch kit. But Exedy's instructions are clear: https://youtu.be/S5xSOy03NAg?t=5m2s They do use an alignment tool as you can see...What do you mean by saying "installing the transmission"? Is there a way to change the clutch without removing the gearbox?

What I understand now, by doing more research, is that the alignment is only necessary for the easier installation of the gearbox. It isn't that you can install the clutch disk in the wrong position, but if it is not aligned correctly you have to do the same job again. Bottom line, if you can install the gearbox without an issue, that means the alignment was correct at the firth place, right?

You'll never get the input shaft to the transmission to go through the disk(s) and into the pilot bearing if you don't align it with the tool before you snug down the pressure plate.

Ok, you might, but you'd be awfully lucky, and you run more of a risk of smacking the pilot bearing in a way you probably don't want to.

I get what you said now. So there is a risk of damaging the pilot bearing during the installation, it's not only that you have to do the same work from the start if you can't install the transmission...

You'll never get the input shaft to the transmission to go through the disk(s) and into the pilot bearing if you don't align it with the tool before you snug down the pressure plate.

Ok, you might, but you'd be awfully lucky, and you run more of a risk of smacking the pilot bearing in a way you probably don't want to.

I get what you said now. So there is a risk of damaging the pilot bearing during the installation, it's not only that you have to do the same work from the start if you can't install the transmission...

What generally happens is you get the trans and engine close to coming together and then they won't go the final little bit like they should. At that point you would stop, pull them back apart and check your alignment. What you don't want to do is start threading the bolts in that hold the engine and trans together and just start cranking thinking they will get them together that last little bit. That is when bad things happen and stuff gets broken. Its best to just use the tool, and make sure everything is aligned. Even then it can sometimes be a bugger to the the two big heavy parts to move together just right and with ease.

What I understand now, by doing more research, is that the alignment is only necessary for the easier installation of the gearbox. It isn't that you can install the clutch disk in the wrong position, but if it is not aligned correctly you have to do the same job again. Bottom line, if you can install the gearbox without an issue, that means the alignment was correct at the firth place, right?

You keep trying to rationalize this but if the clutch disk isn't exactly aligned with the pilot bearing/socket in the flywheel then the two will not mate up properly. It's usually hard enough to get them to mate anyway, if this isn't right then you never will. What's the issue with getting a tool? Didn't one come with the new clutch kit?

You don't need to align the clutch at all if you don't plan on installing the transmission.

Otherwise, I wouldn't skip it.

The car is a Nissan Almera N16 2.2 Di.Is there a way to change the clutch without removing the gearbox?

Not unless you have an old Opel Kadett, on those you could remove the shaft from the rear of the transmission and drop the clutch, pressure plate and throw out bearing out from below with the transmission still in the car, a one hour job. Your car requires the transmission to be removed.